1. Field of the Invention
The present invention relates generally to a mandrel for supporting a tubular workpiece, and more particularly, to such a mandrel for use in a machine which forms heat transfer fins on a tubular member.
2. Description of the Prior Art
The surface areas of various heat transfer components are commonly increased by forming fins on their surfaces. Finned tubular members are used extensively in heat transfer products. For example, one such product in which finned tubular members are employed is a moisture separator-reheater. The most efficient method of finning a tubular member is by roll-forming the surface of the material rather than by cutting away material. A typical apparatus for finning tubular members is the Reed Rolled Thread Die Company Model A23 cylindrical die thread-rolling machine. These machines use three angularly spaced cylindrical dies which rotate in contact with the outer surface of the tubular member and move the material to form fins on the tubular member. The axes of the rotatable dies of the finning machine are slightly skewed to the axis of the tubular member. The rotation of the dies causes the tubular member to rotate, producing helical fins thereon, and to simultaneously feed through the dies.
The pressure of the fin-forming dies could cause the tubular member to distort or collapse. To prevent such distortion a mandrel is commonly employed within the tubular member. Prior to this invention, a mandrel was usually machined from hardened steel and rigidly attached to an arbor. In the process of finning long tubular members up to 60 feet in length, the arbor with rigidly attached mandrel was fixedly mounted to the finning machine. The tubular member rotated on the mandrel and simultaneously slid along the mandrel as the tubular member rotatably fed through the dies. Since the tubular member encountered rotating and sliding frictional forces, the tube-finning process required substantially greater power above that which would be required to process a freely rotating tubular member. It was also found that the heat generated by the rolling and sliding friction of the tube passing over the mandrel caused very short mandrel life, and it was necessary to replace the mandrel several times during each 8-hour work shift.
In another prior art arrangement, the arbor was rotatably mounted to the finning machine. The rotation of the tubular member would cause the arbor with rigidly attached mandrel to rotate with the tubular member. Simultaneously with the mutual rotation, the tubular member would slide along the mandrel. Since the entire arbor with hardened mandrel was rotated with the tubular member, significant additional power was required above that required to rotate the tubular member alone. It was also found that the heat generated by the rolling friction of the arbor and by the friction of the tube passing over the arbor and mandrel caused very short mandrel life. The internal smoothness and diameter of the finned tubular member was found to vary with the wear of the mandrel and it was necessary to replace the mandrel several times during each 8-hour work shift.
By this invention, the disadvantages and limitations of the prior art arrangements are overcome and a mandrel having a lubricated rotatable bushing is provided to facilitate movement of the tubular member during the finning operation. The bushing alone rotates with the tubular member and it is no longer necessary to frictionally rotate the tubular member on the mandrel, or to rotate the entire arbor and mandrel during the finning operation.
Accordingly, a principal object of this invention is to provide a lubricated rotatable mandrel for use with a tube-finning machine, which reduces friction with the tubular member and with mating components to increase the life of the mandrel.
Another object of this invention is to provide a mandrel for use with a tube-finning machine, which reduces the power required to fin the tubular member.
Still another object of this invention is to provide a lubricated rotatable mandrel, for use with a tube-finning machine, which retains a consistent work-supporting surface to thereby consistently maintain the internal smoothness and diameter of the formed tubular members.